34th INTERNATIONAL CONFERENCE ON
PRODUCTION ENGINEERING
28. - 30. September 2011, Niš, Serbia
University of Niš, Faculty of Mechanical Engineering
MANUFACTURABILITY ANALISIS OF DIE-CAST PARTS
Miloš Ristić 1), Miodrag Manić 2), Boban Cvetanović 1)
Higher Technical School of Professiona Studies – Niš, Aleksandra Medvedeva 20, 18000 Nis, Serbia
2)
Faculty of Mechanical Engineering, University of Niš, Aleksandra Medvedeva 14, 18000 Nis, Serbia
milos.ristic@vtsnis.edu.rs, miodrag.manic@masfak.ni.ac.rs, boban.cvetanovic@vtsnis.edu.rs
1)
Abstract: Manufacturability analysis of a product is used at early stages of a design process in order to
asses the possibilities of product realization, reduce the number of design iterations, thus also reducing
the cost. One of the conditions for the automated manufacturability analysis is parametric modeling and
feature-based design. This paper presents the concept of the system for the manufacturability analysis of
die-cast parts. It presents the way to create knowledge basis containing recommendations and
restrictions used for die-casting of a part. The paper also describes advice CA system gives the designer
during the design process by means of which the design process itself is upgraded and the concurrent
engineering environment is created.
Key words: manufacturability analysis, die-casting, part attributes, feature recognition.
1. INTRODUCTION
Die-casting is a method of producing finished castings by
forcing molten metal into a hard metal die, which is
arranged to open after the metal has solidified so that the
casting can be removed (Fig.1).
Fig. 1. Die-casting process terminology
Die-casting is a ‘near net shape’ manufacturing process
extensively used for realizing quality products required in
many engineering applications [1]. Advantages of diecasting process are higher production rate, lower cost,
better quality and process automation. A part to be made
with die-casting should be designed keeping in mind
many process considerations. Involving production
engineers in the product development during the design
phase is the process of concurrent engineering which has
the aim to get critical suggestions and advices related to
part design which is, in fact, manufacturability analysis.
Industries today are striving to achieve lower product
development time, higher productivity and efficiency. In
large enterprises, where design and manufacturing
personnel may be stationed at different locations, the
concept of Design for Manufacture (DFM) is preferred
[2]. Implementing DFM will have the benefits of
improved manufacturability of product design, shorter
time-to-market and reduced cost. Three areas where DFM
can be applied are: Verification; Quantification and
Optimization [3]. Gupta et al [4] have classified DFM into
direct or rule based approaches. In rule based approach,
rules are used to identify design attributes which are
beyond process capabilities, while in direct based
approach the first step is to generate feasible process plan
and to find most suitable plan in order to reduce time or
cost. Shah and Wright [5] have identified DFM metrics
which include qualitative (good practice rules etc.) and
quantitative (cost and time estimates etc.) methods.
Most of the work in DFM has been done in the machining
[4,6] or sheet metal processing [7,8] domain, while little
attention has been given to die casting. In die-casting,
implementation of DFM is important as production lead
times are significantly longer. This is due to greater
number of iterations required between design and
manufacturing teams; die design and manufacturing, and
process simulation and testing are required before
production is started [1]. Certain progress in reducing
production lead time has been achieved in the area of
mold design. [9, 10]. Manufacturability analysis of feature
based model has great importance during virtual product
development [11].
2. FEATURE-BASED DESIGN AS A
PREREQISITE FOR
MANUFACTURABILITY ANALYSIS
Manufacturability analysis requires the application of
feature-based modeling techniques which, besides
geometrical
descriptions,
contain
technological
recommendations and restrictions [12]. Depending on the
manufacturability analysis moment, two approaches can
be defined: analysis during the design process itself (online); analysis done upon the completion of the
constructing process (off-line).
If the term “feature based design” [13] is used and if the
product database, in a specific CAD system, is objectoriented, then we can perform on-line analysis. One of the
3. PROCESS CONSTRAINTS AND DESIGN
GUIDELINES
A part to be die-cast should possess certain design
characteristics to make it suitable for manufacturing with
die- casting process. Following sub-sections elaborate
these constraints and guidelines.
3.2 Overall Part Attributes
Die casting process has limitations on overall part
characteristics, such as part weight, surface area, wall
thickness, material, size, tolerance and surface finish.
These limitations depend on the type of material which
makes it necessary for the designer to evaluate part
against material specific process constraints. Table 1
shows representative database of material specific process
capabilities.
Tab. 1: Die-casting material and process constraints
(source: [15, 16])
Material
Zn
Al
Mg
Weight (kg)
30
45
16
Efective projective area (m2)
0.77 0.77 0.77
Recommended minimum wall thickness (mm)
0.38<25
0.75 - 1.3
0.75
0.7525 – 100
1.3 – 1.8
1.3
1.3100 – 500
1.8 - 2.2
1.8
1.8500 – 2000
2.2 - 2.8
2.2
2.22000 – 5000
2.8 - 6.0
4.6
Minimum wall thickness
6
6
6
Attribute
surface area (cm2)
possible ways to do it is to do it during the constructing
process, i.e. during the process of inserting certain
elements in the product model and to automatically
correct inserted value of a certain parameter if necessary.
Another approach to manufacturability analysis is the
analysis after the design process completion. Using this
approach the whole product model would be analyzed
and, if certain illogicalities are observed the report could
be sent to the designer. This report may take the form of a
warning, or alternatively the model could be changed and
this changed model sent back to the designer. This offline analysis approach is implemented using an expert
system with the expert shell where, as in the case of an
on-line analysis, the product model is object-oriented.
Both analyses are based on parametric design and feature
based project modeling. In order for a feature to be
functional its attributes and characteristics have to be
thoroughly described. Feature attributes carry the
information about specific feature characteristics
important for a current application and they can be
determined at different levels- from the feature level, or
feature set level to the level of describing part or an
assembly [14]. Attributes can also be used to determine
characteristics of a relationship between features and
feature sets. Feature attributes can be position, orientation,
dimensions, shape or tolerancies. Assembly attributes can,
besides other things, contain the information such as:
assembly surface, overlaps/gaps, relative orientation.
Cu
7
0.77
1.52.0
2.02.5
2.53.0
--------6
3.3 Good practice rules
There are certain rules in die-casting part design which
should be followed in order to make a good part.
There are many such good practice rules, some of which
are shown in Figure 3.
3.1 Part Geometry Limitations
Hui [15] has discussed some of the geometric aspects
related to mouldablility of a part. According to Madan et
al [1] some features which cause accessibility problems
are not allowed in die-casting. These are explained in the
following paragraph with the help of figure 2 (a-f):
 Internal undercuts are not allowed in die-casting.
 Features with reverse draft and void features.
 Partially visible depression features like holes
with smaller opening diameter and larger
diameter at the base.
Fig. 3: Good practice rules in die-casting (Source: [16,
17]).
3.4 Manufacturability of Individual Features
Fig. 2: Part geometrical limitations.
Some recommendations regarding individual feature
characteristics in die-casting should be followed in order
to make a good part. Design rules for ribs in die-casting,
limitations on hole diameter and their relationship with
core length, as well as tolerance limitations for a diecasting part, are described in the following sections of this
paper.
4. PART ATTRIBUTES AND FEATURE
RECOGNITION
Feature recognition is a key for automation of any
automated manufacturability evaluation system and that
also applies to die-casting process [1]. Geometric
reasoning or feature recognition rules are applied to get
and store required information of die-casting features.
Feature recognition is done in following domains: Nonmanufacturability features, Features requiring side-cores,
Part attributes, Wall thickness, Sharp edges, and Rib
features.
Any features or regions of the part which pose molding
tool disengagement problems are identified so that same
can be reported to the designer. Side-core diameter and
maximum length limitations depend on the type of alloy
used. Recommended tolerances for die-casting part
[16,17] also depend on material.
During manufacturability analysis of die-casting parts,
determination of parting direction is important for
identifying those die-casting features which require a
side-core for molding.
Overall attributes of the part such as volume, surface area
are directly extracted from the part CAD model, while
tolerance and surface finish evaluation is performed
interactively because of non availability of this data in
machine readable format.
It is important to identify regions of the part which violate
thickness constraints like minimum and maximum
allowed wall thickness and even sharp thickness
variations [18]. It is very critical in die-casting process to
obtain parts with uniform wall thickness and smooth
variations.
Die-casting process requires that, as much as possible,
sharp edges be rounded or smooth, therefore the process
of edges identification is important in the whole
manufacturability analysis assessment. It is important to
identify both sharp edges and smooth edges with
insufficient round radius.
Rib features are those protrusion features in die casting
which have wall thickness comparable to the nominal
wall thickness and much larger length.
Taking into consideration part attributes and feature
recognition, we can create rules and give advices to the
designer. Depending on the available computer aided
systems for manufacturability analysis, we opted to
include necessary rules into parametrically designed gear
housing (fig. 4).
Fig. 4: Parametrically designed reductor housing.
Considering the fact that during the parametric designing
each feature was defined and related, we can include
adequate relations. This was done by defining features
using CATIA V5 (Knowledgeware) software. Using CAx
Template, which contains information and inserted
knowledge necessary to the expert, we define Knowledge
Embedded Template – KET [19]. For example, sharp
corners are undesirable because they become a localized
point of heat and stress built-up in the die steel, which can
cause die cracking and early failure. This is done by using
rounding off sharp edges of the part. Manufacturability
restrictions are directly inserted in parametrically
designed product model in the form of rules. Radius of
this round depends on the wall thickness of the part and is
generally 1.5 times wall thickness. This rule can be of a
great importance when a designer modifies parametrically
defined product. If he wants to change radius user defined
feature at the same time keeping the wall thickness, he
will get an advice stating “radius r2 should be 1.5 times
greater than the wall thickness”.
The rule was inserted in CATIA V5 Knowledgeware in
the following form:
If ‘Fillet_Radius_Value’ < ‘1.5*Wall_Thickness’
{
Message (“Radius should be 1.5 times greater
than the wall thickness !
Modify a parameter according to the rule.”)
}
And as such was tested at a redactor housing. The figure 5
presents the advice in the form of information given by
knowledgeware to a user who wants to change the
described radius outside restrictions.
Fig. 5: Knowledgeware gives advice on the basis of
previously defined rule.
Knowledge based technologies enable a user to define
object-oriented product model and to include material
characteristics, technological process requirement,
standards and rules necessary for rule based design. It is
important to note that these rules are not unchangeable,
but can be customized to a user.
If we use relational dependency to connect, for example,
rib thickness with the wall thickness and set the
requirement using if/then relations to emphasize that the
rib thickness has to be equal to the wall thickness, we get
manufacturing restriction that a designer will use as an
advice during product designing (Tab. 2).
Previously stated rule, along with some other rules, were
described in table 2 where manufacturability evaluation of
each rule was also described. In addition, adequate
advices were given in order to help designer during the
continuation of product development.
Tab. 2: Part manufacturability evaluation and advice.
Part attribute /
feature
Weight: 4.12 kg
Maximum wall
thickness: 30mm
Number of sidecores: = 4
Rib with = 8mm
Sharp edges
Manufacturability evaluation and
advice
Is within limits of process.
Maximum wall thickness should be
reduced below 6.3 mm.
Number of side-cores required is high
and should be reduced.
Thickness of the rib is high and should
be made equal to wall thickness.
Sharp edges in the part should be
rounded or smoothed.
Nowadays, different methods [20] are developed for
assessing manufacturability of parts created during diecasting process.
5. CONCLUSION
Specificities of die casting processes and the available
resources have to be taken into consideration during
manufacturability analysis and presented to the designer
in the form of either on-line or off-line advices, depending
on the chosen process chosen. Previously presented online process is parametrically modeled part with inserted
rules according to the if/then relations. The advice
designer gets is the result of the set of experiential rules
(examples of good practices) inserted during previous
phrases of project design. Advice received aims at final
product being produced more easily, distributed more
cheaply, at the same time following the product life-cycle.
In general, working with knowledgeware systems
provides control and monitoring reduction during
designing process and unites product development phases
raising a concurrent engineering to a higher level, which
is extremely important in terms of manufacturability.
ACKNOWLEDGEMENT
This research is sponsored by the Ministry of Science and
Technology of the Republic of Serbia - project id III
41017 for the period of 2011-2014.
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